not able to reveice emails Remote_host_said:_554_5.7.1

Hello,

I'm running a zimbra server for over 2 months now. The Zimbra server is accepting mails on one domain (domain1.com). The Zimbra server is in the MX record of domain1.com. Everything with domain1 is working fine. The Zimbra server is receiving all the mails.

Now I put the Zimbra Server in the MX records of domain2.com which is also on the zimbra server and I tried to send mails from outside.

I get the following Mail Delivery failed message:
[IP_of_Zimbra_server]_does_not_like_recipient./Remote_host_said:_554_5.7.1_<user@domain2.com>:_Re lay_access_denied/Giving_up_on_[IP_of_Zimbra_server]./

The Account user@domain2.com is existing on the zimbra server. So I don't know why the zimbra server is rejecting the mails to domain2.com